It is well known in the art that extra fuel, that is, acceleration fuel, is supplied to the suction channel in response to the rapidly increasing quantity of suction air when the degree of opening of the throttle valve is enlarged to increase the rotational speed of the engine.
A carburetor, which is one type of fuel supply system, may comprise an acceleration apparatus having a piston-type (or a diaphragm-type) acceleration pump linked to a throttle valve, for example, a carburetor for an automobile engine, as cited in Japanese Patent Application Laid-open No. 53-81831. This acceleration device delivers and supplies acceleration fuel to the suction channel by transmitting the rotation of the throttle valve to the piston (or diaphragm) of the acceleration pump by means of a link mechanism.
Despite the fact that the acceleration apparatus described above is easily applied to a carburetor having a butterfly-type throttle valve, providing the link mechanism described above to the exterior of the carburetor main body in a carburetor for general-purpose engines not only unavoidably results in greater complexity and significantly greater size, which is contrary to basic needs and characteristics that the structure be small and simple, but also makes it essentially impossible to apply such an apparatus to a rotating throttle valve-type carburetor often used in general-purpose engines, because the throttle valve moves in the axial direction while rotating.
As a result, it has been proposed that an acceleration apparatus be provided to a rotating throttle valve-type carburetor, as cited in Japanese Utility Model Application Laid-open No. 6-67842, such that a piston-type acceleration pump is disposed at a right angle to the throttle valve stem. The piston is operated by a cam formed on the external peripheral surface of the throttle valve to deliver and supply acceleration fuel to the suction channel.
The acceleration apparatus provided to the rotating throttle valve-type carburetor described above does not have a complex link mechanism on the exterior to change the rotating movement of the throttle valve to the linear movement of the acceleration pump. However, very troublesome fabrication is required in that a cam in the form of a groove extending in the circumferential direction must be formed on the external peripheral surface of the cylindrical throttle valve so that the acceleration fuel corresponds to the quantity of the suction air. The cam in the form of a groove additionally raises concern that the airtightness of the external peripheral surface of the throttle valve may be impaired, and that air may be sucked into the suction channel during idling depending on the formation site, resulting in a malfunction in idling.